Messages in this thread | | | From | "Doug Smythies" <> | Subject | RE: [RFT][PATCH v5 7/7] cpuidle: menu: Avoid selecting shallow states with stopped tick | Date | Tue, 20 Mar 2018 10:15:02 -0700 |
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On 2018.03.20 11:22 Doug Smythies wrote: > On 2018.03.19 05:47 Thomas Ilsche wrote: >> On 2018-03-15 23:19, Rafael J. Wysocki wrote: >>> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com> >>> >>> If the scheduler tick has been stopped already and the governor >>> selects a shallow idle state, the CPU can spend a long time in that >>> state if the selection is based on an inaccurate prediction of idle >>> time. That effect turns out to be noticeable, so it needs to be >>> mitigated. >> >> What are some common causes for that situation? >> How could I trigger this for testing? > > It appeared quite readily with my simple 100% load > on one CPU test. Back then (V3) there only 6 patches in the set, > and before the re-spin there ended up being a patch 7 of 6, which > made a significant difference in both package power and the > histograms of times in each idle state. > > Reference: > https://marc.info/?l=linux-pm&m=152075419526696&w=2
I made a kernel (4.16-rc5) with only patches 1 to 6 of 7 (V6) and also with the poll fix.
I took an old graph: http://fast.smythies.com/rjwv3pp_100.png
and removed an obsolete line and added a line from this kernel:
http://fast.smythies.com/rjwv6m_100.png
I also acquired a trace during the test and observe:
Report: Summary:
Idle State 0: Total Entries: 699 : PowerNightmares: 0 : Not PN time (seconds): 0.031169 : PN time: 0.000000 : Ratio: 0.000000 Idle State 1: Total Entries: 3855 : PowerNightmares: 106 : Not PN time (seconds): 0.123759 : PN time: 43.511914 : Ratio: 351.585856 Idle State 2: Total Entries: 3688 : PowerNightmares: 181 : Not PN time (seconds): 1.303237 : PN time: 63.241424 : Ratio: 48.526418 Idle State 3: Total Entries: 528 : PowerNightmares: 115 : Not PN time (seconds): 0.276290 : PN time: 44.764111 : Ratio: 162.018571
Where "PowerNightmare" is defined as spending excessive time in an idle state, and arbitrarily defined for my processor as:
#define THRESHOLD_0 100 /* Idle state 0 PowerNightmare threshold in microseconds */ #define THRESHOLD_1 1000 /* Idle state 1 PowerNightmare threshold in microseconds */ #define THRESHOLD_2 2000 /* Idle state 2 PowerNightmare threshold in microseconds */ #define THRESHOLD_3 4000 /* Idle state 3 PowerNightmare threshold in microseconds */
While this trace file was only about 15 megabytes, I have several 10s of gigabytes of trace data for V4 + poll fix and never see any excessive time spent in any idle state.
... Doug
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